The invention proceeds from electrolysis processes known per se for electrolysis of aqueous alkali metal chloride solutions using oxygen-consuming electrodes in the form of gas diffusion electrodes which typically comprise an electrically conductive carrier and a gas diffusion layer comprising a catalytically active component.
Various proposals for operation of the oxygen-consuming electrodes in electrolysis cells on the industrial scale are known in principle from the prior art. The basic idea is to replace the hydrogen-evolving cathode in the electrolysis (for example in chloralkali electrolysis) with the oxygen-consuming electrode (cathode). An overview of the possible cell designs and solutions can be found in the publication by Moussallem et al “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
The oxygen-consuming electrode—also called OCE for short hereinafter—has to meet a series of requirements to be usable in industrial electrolyzers. For instance, the catalyst and all other materials used have to be chemically stable against concentrated alkali metal hydroxide solutions and towards pure oxygen at a temperature of typically 80-90° C. Similarly, a high degree of mechanical stability is required, such that the electrodes can be installed and operated in electrolyzers with a size typically more than 2 m2 in area (industrial scale). Further desirable properties are: high electrical conductivity, low layer thickness, high internal surface area and high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and a corresponding pore structure for transmission of gas and electrolyte are likewise necessary, as is such imperviosity that gas and liquid space remain separate from one another. Long-term stability and low production costs are further particular requirements on an industrially usable oxygen-consuming electrode.
A problem in the case of arrangement of an OCE in a cathode element arises from the fact that, on the catholyte side, the hydrostatic pressure forms a gradient over the height of the electrode, which is opposed on the gas side by a constant pressure over the height. The effect of this can be that, in the lower region of the electrode, the hydrophobic pores too are flooded and liquid gets onto the gas side. On the other hand, in the case of excessively high gas pressure in the upper part of the OCE, liquid can be displaced from the hydrophilic pores and oxygen can get onto the catholyte side. Both effects reduce the performance of the OCE. In practice, the effect of this is that the construction height of an OCE is limited to about 30 cm unless further measures are taken.
A preferred solution to this problem results from an arrangement in which the catholyte is conducted from the top downward through a flat porous element mounted between OCE and ion exchange membrane, called a percolator, in a kind of free-falling liquid film, called falling film for short, along the OCE. In this arrangement, no liquid column bears on the liquid side of the OCE, and no hydrostatic pressure profile builds up over the construction height of the cell. A description of this arrangement can be found in WO 2001/57290 A1.
In another version, the ion exchange membrane which, in the electrolysis cell, divides the anode space from the cathode space, without an intervening space for the flow of an alkali, called catholyte gap for short, directly adjoins the OCE. This arrangement is also referred to as the “zero gap” arrangement, as opposed to a “finite gap” arrangement in which the alkali metal hydroxide solution is conducted through a defined narrow gap between OCE and the membrane. The zero gap arrangement is typically also employed in fuel cell technology. A disadvantage here is that the alkali metal hydroxide solution which forms has to be passed through the OCE to the gas side and then flows downwards at the OCE. In the course of this, the pores in the OCE must not be blocked by the alkali metal hydroxide, and there must not be any crystallization of alkali metal hydroxide in the pores. It has been found that a very high alkali metal hydroxide concentration can indeed arise here too, but it is stated that the ion exchange membrane at these high concentrations lacks long-term stability (Lipp et al., J. Appl. Electrochem. 35 (2005)1015—Los Alamos National Laboratory “Peroxide formation during chlor-alkali electrolysis with carbon-based ODC”).
An oxygen-consuming electrode consists typically of a support element, for example a plate of porous metal or a metal wire mesh, and an electrochemically catalytically active coating. The electrochemically active coating is microporous and consists of hydrophilic and hydrophobic constituents. The hydrophobic constituents make it difficult for electrolytes to penetrate through and thus keep the corresponding pores in the OCE unblocked for the transport of the oxygen to the catalytically active sites. The hydrophilic constituents enable the electrolyte to penetrate to the catalytically active sites, and the hydroxide ions to be transported away from the OCE. The hydrophobic component used is generally a fluorinated polymer such as polytetrafluoroethylene (PTFE), which additionally serves as a polymeric binder for particles of the catalyst. In the case of electrodes with a silver catalyst, for example, the silver serves as a hydrophilic component.
A multitude of compounds have been described as electrochemical catalysts for the reduction of oxygen. However, only platinum and silver have gained practical significance as catalysts for the reduction of oxygen in alkaline solutions.
Platinum has a very high catalytic activity for the reduction of oxygen. Due to the high costs of platinum, it is used exclusively in supported form. A preferred support material is carbon. However, stability of carbon-supported and platinum-based electrodes in long-term operation is inadequate, probably because platinum also catalyses the oxidation of the support material. Carbon additionally promotes the unwanted formation of H2O2, which likewise causes oxidation. Silver likewise has a high electrocatalytic activity for the reduction of oxygen.
Silver can be used in carbon-supported form, and also as fine metallic silver. Even though the carbon-supported silver catalysts are more durable than the corresponding platinum catalysts, the long-term stability thereof under the conditions in oxygen-consuming electrodes, especially in the case of use for chloralkali electrolysis, is limited.
In the case of production of OCEs comprising unsupported silver catalyst, the silver is preferably introduced at least partly in the form of silver oxides, which are then reduced to metallic silver. The reduction is generally effected when the electrolysis cell is first started up. The reduction of the silver compounds also results in a change in the arrangement of the crystals, more particularly also to bridge formation between individual silver particles. This leads to overall consolidation of the structure.
It has been observed that, when the electrolysis current is switched off, the silver catalyst can be oxidized again. The oxidation is apparently promoted by the oxygen and the moisture in the half-cell. The oxidation can result in rearrangements in the catalyst structure, which have adverse effects on the activity of the catalyst and hence on the performance of the OCE.
It has also been found that the performance, especially the electrolysis voltage required, in an OCE with a silver catalyst depends considerably on the startup conditions. This applies both to the first startup of an OCE and to the further startups after a shutdown. It is one of the objects of the present invention to find specific conditions for the operation and especially the startup of an OCE with a silver catalyst, which ensure a high performance of the OCE.
A further central element of the electrolysis cell is the ion exchange membrane. The membrane is pervious to cations and water and substantially impervious to anions. The ion exchange membranes in electrolysis cells are subject to severe stress: They have to be stable towards chlorine on the anode side and to severe alkaline stress on the cathode side at a temperature around 90° C. Perfluorinated polymers such as PTFE typically withstand these stresses. The ions are transported via sulphonate groups or carboxyl groups polymerized into these polymers. Carboxyl groups exhibit higher selectivity, have lower water absorption and have higher electrical resistance than sulphonate groups. In general, multilayer membranes are used, with a thicker layer containing sulphonate groups on the anode side and a thinner layer containing carboxyl groups on the cathode side. The membranes are provided with a hydrophilic layer on the cathode side or both sides. To improve their mechanical properties, the membranes are reinforced by the inlaying of wovens or knits; the reinforcement is preferably incorporated into the layer containing sulphonate groups.
Due to the complex structure, the ion exchange membranes are sensitive to changes in the media surrounding them. Different molar concentrations can result in formation of significant osmotic pressure gradients between the anode and cathode sides. When the electrolyte concentrations decrease, the membrane swells as a result of increased water absorption. When the electrolyte concentrations increase, the membrane releases water and shrinks as a result; in the extreme case, withdrawal of water can cause precipitation of solids in the membrane or mechanical destruction of the membrane.
Concentration changes can thus cause disruption and damage at the membrane. The result may be delamination of the layer structure (blister formation), as a result of which the mass transfer through the membrane deteriorates.
In addition, pinholes and, in the extreme case, cracks can occur, which can result in mixing of anolyte and catholyte.
In production plants, it is desirable for electrolysis cells to be operated over periods of up to several years, without opening them in the meantime. Due to variation in demand volumes and faults in production sectors upstream and downstream of the electrolysis, electrolysis cells in production plants, however, inevitably have to be repeatedly switched off and back on again.
On shutdown and restart of the electrolysis cells, there occur conditions which can lead to damage to the cell elements and considerably reduce the lifetime thereof. More particularly, oxidative damage has been found in the cathode space, as have damage to the OCE and damage to the membrane.
The prior art discloses few modes of operation with which the risk of damage to the electrolysis cells in the course of startup and shutdown can be reduced.
A measure known from conventional membrane electrolysis is the maintenance of a polarization voltage, which means that, when the electrolysis is ended, the potential difference is not run down to zero, but maintained at the level of the polarization voltage. In practical terms, a somewhat higher voltage than that required for the polarization is set, such that a constant low current flows and electrolysis proceeds to a minor degree. However, in the case of use of OCEs, this measure is insufficient to prevent oxidative damage to OCEs which have been shut down.
Published specification JP 2004-300510 A describes an electrolysis process using a micro-gap arrangement, in which corrosion in the cathode space is to be prevented by flooding the gas space with sodium hydroxide solution on shutdown of the cell. The flooding of the gas space with sodium hydroxide solution accordingly protects the cathode space from corrosion, but gives inadequate protection from damage to the electrode and the membrane on shutdown and startup, or during shutdown periods.
U.S. Pat. No. 4,578,159A1 states that, for an electrolysis process using a zero gap arrangement, purging the cathode space with 35% sodium hydroxide solution prior to startup of the cell, or starting up the cell with low current density and gradually increasing the current density, prevents damage to membrane and electrode. This procedure reduces the risk of damage to membrane and OCE during startup, but does not give any protection from damage during shutdown and shutdown periods.
Document U.S. Pat. No. 4,364,806A1 discloses that exchange of the oxygen for nitrogen after downregulating the electrolysis current will prevent corrosion in the cathode space. According to WO2008009661A2, the addition of a small proportion of hydrogen to the nitrogen will give rise to an improvement in protection from corrosion damage. The methods mentioned, however, are complex and entail the installation of additional equipment for nitrogen and hydrogen supply. Moreover, the addition of hydrogen increases the safety risk in the course of operation of such electrolysers through formation of explosive gas mixtures, since residues of oxygen may be present in the cathode space. On restart, the pores of the OCE are partly filled with nitrogen, which prevents the supply of oxygen to the reactive sites. The process also does not give any protection from damage to the ion exchange membrane.
The Final Technical Report “Advanced Chlor-Alkali Technology” by Jerzy Chlistunoff (Los Alamos National Laboratory, DOE Award 03EE-2F/Ed190403, 2004) details conditions for the temporary shutdown and startup of zero gap cells. In the case of shutdown, after the electrolysis current has been stopped, the oxygen supply is stopped and replaced by nitrogen. The moistening of the gas stream is increased in order to wash out the remaining NaOH. On the anode side, the brine is replaced by hot water (90° C.). The procedure is repeated until a stable polarization voltage (open-circuit voltage) has been attained. The cells are then cooled, then the supply of moist nitrogen and the pumped circulation of the water on the anode side are stopped.
For the restart, the anode side is first filled with brine; on the cathode side, water and nitrogen are introduced. The cell is then heated to 80° C. Then the gas supply is switched to oxygen and a polarization voltage with low current flow is applied. Subsequently, the current density is increased and the pressure in the cathode is increased; the temperature rises to 90° C. Brine and water supply are subsequently adjusted such that the desired concentrations on the anode and cathode sides are attained.
The known processes described are complex to conduct; this is especially true of industrial electrolysis plants, where safety aspects are of increased importance. Moreover, not all processes can be applied to electrolysis cells with a finite gap arrangement.
It should be stated that the techniques described to date for startup and shutdown of an OCE are disadvantageous and give only inadequate protection from damage.
It is an object of the present invention to provide an improved electrolysis process for chloralkali electrolysis using an OCE in the finite gap arrangement with suitable operating parameters for startup and shutdown of the electrolysis cell having an OCE with a silver catalyst as the electrocatalytic substance, which are simple to perform and where compliance prevents damage to membrane, electrode and/or other components of the electrolysis cell.